If you haven't taken care of filing your income tax return yet, you have a few extra days to so this year. The traditional deadline to get those returns postmarked is April 15th, but it does not apply when the date falls on a weekend, a federal holiday or a holiday celebrated in Washington, DC.
It's the latter that has pushed tax day to Monday April 18th this time around. Emancipation Day is a holiday in the District of Columbia, and it normally falls on April 16th. Since that's a Saturday this year, it is moved back to Friday April 15th, which shoves the tax deadline to the following Monday.
In Maine and Massachusetts, you get even more time, as Patriot's Day is the 18th, meaning residents of those states have until April 19th to pay Uncle Sam.